PROTEINS 119 



appear to control hereditary characteristics. The mechanism of this 

 control is of intense interest but so far has not been elucidated. Re- 

 lated nucleoproteins called viruses are the infectious agents inducing 

 many common diseases in plants and animals. 



Animals are characterized in part by their extensive nutritional re- 

 quirements and have less synthetic ability than do green plants and 

 microorganisms. Proteins or at least certain essential amino acids and 

 perhaps peptides must be available for the well being of animals. 

 Protein metabolism and nutrition are discussed on page 463 along with 

 requirements for particular amino acids. 



The velocities of biological processes are subject to indirect controls 

 of specific types in at least the moderately complex forms. The sub- 

 stances involved act mainly on the enzymes catalyzing the reactions 

 affected and are called hormones. Hormones of several chemically dif- 

 ferent classes, including proteins and peptides, are known. They are 

 considered later in connection with the regulation of the growth of 

 plants and of various physiological functions of animals. 



Finally, proteins are known to be toxic to animals under particular 

 conditions. Usually toxicity arises only when proteins are transferred 

 from one species to another, without opportunity for denaturation or 

 digestion. Venoms of snakes and insects are injected by biting into the 

 tissues or circulatory systems. Certain of these foreign proteins are 

 actually enzymes that attack components of the body, producing dis- 

 turbances that may result in death. Destruction of blood cells and 

 proteins is typical of the action of the proteins in snake venoms. In 

 other cases, as with certain bacterial toxins, the proteins may prevent 

 the function of special nerve circuits. The toxin from Clostridium 

 botulinum blocks the action of the nerve of the diaphragm when in- 

 jected into a mammal. 



Other toxicities involving foreign proteins result from responses by 

 the affected animal. Allergic reactions fall into this category and occur 

 when an animal is exposed to a foreign protein to which it has been 

 sensitized by prior exposure. The extent and severity of the symptoms 

 depend upon the nature of the foreign protein, the mode of ad- 

 ministration, the quantity given, and the animal species concerned. 

 The toxic disturbances range from respiratory congestion, skin blis- 

 ters, and swelling of limbs to unconsciousness or death. The symp- 

 toms depend upon physiological changes induced by the original sensi- 

 tizing exposure and occur on re-exposure to the same foreign pro- 

 tein. Owing to its medical importance, this problem has been 

 intensively studied, but the mechanism of the process is still unknown. 



